The Neo4j Manual v2.2.0-M0438.6. Update

38.6. Update

Important

To update an index entry, the old one must be removed and a new one added.
For details on removing index entries, see Section 38.5, “Remove”.

Remember that a node or relationship can be associated with any number of key-value pairs in an index.
This means that you can index a node or relationship with many key-value pairs that have the same key.
In the case where a property value changes and you’d like to update the index, it’s not enough to just index the new value — you’ll have to remove the old value as well.